The present invention relates to a semiconductor device employing a III-V nitride semiconductor, and more particularly, to a transistor for use in a high frequency device.
III-V nitride semiconductors, i.e., gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and a mixed crystal material represented by a general formula (InxAl1-y)yGa1-yN where 0≦x≦1 and 0≦y≦1, have a wide band gap and a direct transition band structure. Not only applications of III-V nitride semiconductors to optical devices which utilize such physical features, but also applications thereof to electronic devices which utilize a large breakdown field and a large saturated electron velocity, have been studied. Particularly, a Hetero-junction Field Effect Transistor (hereinafter abbreviated as HFET) which employs 2 Dimensional Electron Gas (hereinafter abbreviated as 2DEG) occurring at an interface between AlxGa1-xN and GaN which are epitaxially grown on a semi-insulating substrate, is being developed as a high power and high frequency device.
In order to improve element characteristics of these nitride semiconductor elements, it is necessary to reduce parasitic resistance components in the semiconductor element, such as contact resistance, channel resistance, and the like, to the extent possible. When current is transported by electrons, it is necessary to externally form an ohmic contact in a region in which the electrons are conducted.
As a conventional ohmic contact, for example, a multilayer metal thin film is used in which aluminum (Al), nickel (Ni), gold (Au), or the like is laminated on titanium (Ti; lowermost layer) formed on a nitride semiconductor layer of AlGaN or the like.
After a multilayer metal thin film having the Ti lowermost layer on the nitride semiconductor layer is formed, a heat treatment is performed at about 500° C. to about 900° C. so that Ti in the multilayer metal thin film and nitrogen (N) react with each other in the vicinity of an upper surface of the nitride semiconductor layer. N is extracted out by the reaction, so that holes are formed in the region in the vicinity of the upper surface of the nitride semiconductor layer. Therefore, the metallicity of the region in the vicinity of the upper surface of the nitride semiconductor layer increases. Also, the reaction of Ti and the nitride semiconductor generates Ga, Al, Ti, and a compound, such as TiN or the like. These products react with Al, Ni, Au, or the like in the multilayer metal thin film, thereby forming a more stable metal compound. As a result, a low-resistance ohmic contact is obtained.
By employing a contact layer made of GaN or the like which is doped to n type to the extent possible with respect to the nitride semiconductor layer in which an ohmic electrode is formed, the contact can be further reduced.
Alternatively, a method of improving electron concentration by providing a contact layer having a superlattice structure made of n type-doped AlGaN and GaN, has been proposed (see, for example, Japanese Unexamined Patent Publication No. 2005-26671, Japanese Unexamined Patent Publication No. H09-172164, and Japanese Unexamined Patent Publication No. H11-121472).
However, when the above-described conventional ohmic contact is a contact layer made of n type-doped GaN or the like, the lower limit of the contact resistance is determined by the activation rate of an impurity in the contact layer (5×1018 cm−3 to 3×1019 cm−3). This is because the highest carrier concentration of the contact layer is determined by the impurity activation rate.
Also, when a superlattice made of n type-doped AlGaN and GaN is used as a contact layer, since an impurity is also doped into an interface between AlGaN and GaN where electrons are accumulated, scattering of electrons occurs due to the impurity, resulting in a reduction in electron mobility. As a result, the contact resistance and the parasitic resistance cannot be sufficiently reduced.
Also, when the n type ohmic electrode obtained by using this technique is used as a source electrode and a drain electrode of an HFET, negative piezoelectric charge occurs at an interface between an electron traveling layer of the HFET made of AlGaN or the like and the superlattice. Potential barrier for electrons at an interface between the high-concentration doped GaN layer of the superlattice and AlGaN of the electron traveling layer increases due to the negative piezoelectric charge, resulting in an increase in the contact resistance and the parasitic resistance.